This invention relates generally to transport refrigeration systems and, more particularly, to a method and apparatus for sensing and correcting a reverse motor condition when a transport refrigeration system is operating in a stand-by mode.
In a transport refrigeration system, such as a container, truck or truck trailer, for example, the power to operate the compressor and the fan motors of the refrigeration system is derived from a generator or alternator that is driven by the prime mover, i.e. the truck's engine. However, when the truck's engine is shut down, such as when it has reached its destination and waiting to be unloaded, for example, an auxiliary or a stand-by system at the site is relied on to provide that power.
One problem that may occur when operating in stand-by power is that of a phase reversal, such that the electric motors are driven in the wrong direction. This results from that fact that the phase relationships may be reversed from one facility to another, such that a motor driven by the stand-by power may be caused to operate in the proper direction but may, just as well, be caused to operate in a reversed direction. If this occurs, then the motor driven equipment, such as the compressor, a condenser fan or an evaporator fan will not operate efficiently.
One approach that has been employed in refrigerated containers wherein a scroll compressor is used, is that of sensing a pressure differential across the compressor to determine whether it is being driven in the proper direction. While this approach is satisfactory for systems with a scroll compressor, it is not effective when using reciprocating compressors since they have negligible pressure differential between correct and incorrect phasing.
An alternative approach that has been used is to provide a dedicated electronic module to sense and correct phasing during two-phase stand-by operation. With this approach, the electronic module operates to sense the voltage drop across two of the three legs of the three-phase motor to see which phase is leading the others. While this approach is effective, it requires the use of a dedicated module, with its attendant manufacturing and reliability expense.